1. Field of the Invention
The present invention relates to a method for effectively measuring a packet delay in an asynchronous packet transfer network. More particularly, the present invention relates to a method for measuring an end-to-end delay, and asynchronous packet transmitter and receiver, capable of conveniently and exactly measuring a packet delay in an asynchronous packet transfer network in which a transfer priority is determined as in MPLS (Multi-Protocol Label Switching).
2. Discussion of Related Art
With tremendous distribution of Internet services, many users have a demand for a network structure that guarantees quality of service (QoS) and safety, as well as an end-to-end connection. This QoS-guaranteed service implementation cannot be provided by an existing Internet structure. In order to support this, various schemes are actively worked out in international and national standardization groups. Particularly, Focus Group for Next Generation Network (FGNGN) of the ITU Telecommunication Standardization Sector (ITU-T) defines an Internet-based circuit oriented packet switching structure of NGN. A fundamental premise of the newly defined network structure is to guarantee QoS for the services coming into the network. To the end, a session- or flow-based admission control function is essential. In other words, before one session is initiated, it should be determined whether to meet the QoS required by the session, and determined whether to admit the session.
The QoS in the packet switching mode is estimated based on basic measurement values of packet traffic: loss, delay, and delay variation of the packet. Thus, measurement and monitoring of these values become important in checking whether the QoS is successfully guaranteed. The present invention proposes a method for effectively measuring the delay value of a packet in equipment.
The delay measuring method is being actively studied, and its standardizing work is in progress mainly at MEF (Metro Ethernet Forum) or ITU-T as well. A delay measuring scheme that is used currently is designed to allocate and transfer the value of a time stamp to a transmission packet in the network where all network facilities are synchronized to the same time, because all network facilities are synchronized like SONET (Synchronous Optical NETwork) or SDH (Synchronous Digital Hierarchy). For example, a method is used which measures the delay of a packet or calculates a round-trip delay between two facilities when not synchronized, to infer that the measured or calculated half value is a one-way delay.
However, the conventional delay measuring method has the following problems. In the case where the time stamp is used, it is premised on the synchronization of the network. Hence, it is difficult to apply to an asynchronous network such as Ethernet. An NPT (Network Timing Protocol) may apply which is proposed to synchronize between network facilities in an IP layer. The NTP provides accuracy in a microsecond in the case of synchronization in a narrow area network such as LAN (Local Area Network), but provides only accuracy in a millisecond in a wide area network such as WAN (Wide Area Network). For this reason, the NPT cannot extend to time synchronization of a large-scaled transport network. Further, when the NPT extends to the large-scaled network, a system load for NPT processing between the facilities is increased, so that the facilities can be deteriorated in performance.
When the delay value is inferred using the round-trip delay value, a time, which it takes to transmit a measurement packet from a source node to a destination node and then to receive its response, is measured, and thereby a delay between the source node and the destination node is measured. In this case, the delay value is inferred by taking the half of the round-trip delay value on the assumption that the traffic is equally distributed in both directions. As such, the delay value cannot be exactly measured in an actual network circumstance where the bi-directional traffic distributions are not equal.
Meanwhile, in order to solve a problem, namely a QoS issue, of the traditional packet switching scheme, the circuit oriented packet switching scheme is used. Thereby, it is possible for network operators to control a path of the traffic. The packets are switched at all nodes based on a destination address of the packet in the existing packet network. However, in the circuit-based case, there is provided a traffic engineering function capable of choosing a different path according to a destination node as well as a source node or service type. In other words, the circuit oriented technique operates the network in such a manner that a different path is used for packets having the same destination depending on the source node or traffic characteristics. This can maximize efficiency of the network.
The most typical circuit oriented packet switching scheme is MPLS. The MPLS is adapted to transmit multiple types of packets through a single path. The MPLS can also provide differentiated service with respect to multiple classes of traffic. Typical class-specific traffic engineering using the MPLS can be generally classified into two: L-LSP (Label-Only-Inferred-PSC Label Switched Path) and E-LSP (EXP-Inferred-PSC Label Switched Path). The two schemes control a priority or class of traffic in a different fashion. In the case of the L-LSP, all traffics delivered through one label are provided with a same class of service. In the case of the E-LSP, traffics delivered through the same label are provided with a different class of service. In other words, in the L-LSP, the traffic class is indicated in a label value, whereas in the E-LSP, the traffic class is indicated in an Exp field rather than the label value of an MPLS header.
The packets delivered through the pre-defined MPLS path monitor and manage safety of the transfer path through OAM (Operation, Adminmstration and Maintenance) functions which are mainly provided on the basis of the path. One of important functions of the OAM is to measure performance of the transfer path. Among performance of the transfer path, delay information is important one of informing the state of a network. A lot of efforts have been made to measure the delay information of a packet in a virtual circuit-based MPLS network, but most of the efforts have been made by the time stamp. In this case, support in the asynchronous network is difficult due to network synchronization. Another way is to use a delay time. However, since the MPLS is for uni-directional transmission and does not have the same delay in both directions, the use of the delay time is not appropriate. Thus, there is a strong need for an efficient delay measuring method for measuring a uni-directional delay of a virtual circuit like the MPLS while being available in a gradually increasing asynchronous network such as Ethernet.